In the past many different techniques have been utilized to combine the outputs of millimeter (mm) wavelength solid state devices to obtain a single high power signal. These power combining techniques have included such apparatus as nonresonant hybrid combiners, nonresonant N-way combiners and resonant N-way combiners. Power combining at mm frequencies has evolved from an extension of techniques developed at the lower microwave frequencies by scaling down the microwave hardware to accommodate the mm wavelengths. The most commonly used combining technique at mm wavelengths has been the resonant N-way combiner comprising an enclosed rectangular cavity which combines the outputs of N discrete oscillators, which can be mounted in the cavity walls. These prior art combiners are seriously limited in power output and combining efficiency in this spectral region because of size and volume restrictions thereon necessary to achieve mode separation and to avoid power losses due to multimoding. This follows from the fact that in a closed cavity resonator the number of resonant modes within a given volume is directly proportional to the resonator volume and to the square of the frequency. Consequently, as the frequency increases to the mm region, the mode density increases, mode separation decreases and thus excitation at a single frequency becomes more difficult.
The Gunn and IMPATT devices often used as the active elements for oscillators at mm wavelengths have a negative resistance over a wide frequency range and thus an enclosed cavity used to combine the outputs of such devices must have low mode density and a small volume relative to the operating wavelength if multimoding is to be avoided. This size limitation restricts the number of such oscillator outputs which can be combined with such resonators, and produces fabrication difficulties.
The present invention avoids many of these disadvantages of the prior art by utilizing an open resonator of the type which has heretofore only been used in laser devices at optical wavelengths.